This invention relates to a monitoring apparatus which measures the concentration of dioxins and related compounds such as dioxin precursors in flue gas or the atmosphere by detecting dioxins and related compounds present in combustion gases from incineration of domestic waste and industrial waste, gases from metal refineries, automobile exhaust or the atmosphere. It relates also to a combustion controller which efficiently employs the results of monitoring in combustion.
When waste is incinerated in a garbage incineration plant, highly toxic dioxins are produced in the flue gas. This gives rise to environmental pollution and is a serious social problem.
Dioxins are toxic to humans in various ways. Not only do they have acute toxicity, but they are also carcinogenic and teratogenic, and recently, it has been shown that they act as xe2x80x9cenvironmental hormonesxe2x80x9d, false hormones which disturb the internal secretions of the body. Dioxins are also known to be discharged in waste gases from metal refining, exhaust from automobiles, or lye from bleaching processes.
The term xe2x80x9cdioxinsxe2x80x9d is a general term referring to 75 isomers of polychlorinated dibenzene paradioxin (PCDDS) and 135 isomers of polychlorinated dibenzofuran (PCDFs), and in the wider sense includes polybisphenyl chlorides (Coplanar PCBS). Hereafter, dioxin and related compounds will be referred to simply by the general term xe2x80x9cdioxinsxe2x80x9d.
Although a great deal is known about the mechanisms by which dioxins are produced (xe2x80x9cBunsekixe2x80x9d, 1998, pp. 512-519), the conditions under which this occurs vary widely depending on location and mechanism, and are very complex. One of the leading factors is considered to be reaction between carbon and chlorine (de novo or new product synthesis) due to metal chlorides of cobalt, iron and copper which are present in the ash of combustion processes under the high temperature of incineration plants, and which act as catalysts. In the basic reaction of this de novo synthesis, when carbon atoms, chlorine atoms and oxygen atoms are present together at high temperature, they produce many organochlorine compounds such as dioxins, chlorobenzene and chlorophenol by radical reactions. It is said that this chlorobenzene and chlorophenol are themselves precursors of, and give rise to, dioxins. Formation of dioxins in a waste incineration plant is said to mainly occur in two places, i.e., a process which takes place during incomplete combustion in an incinerator when the incineration temperature is less than 800xc2x0 C., and in a de novo synthesis in a boiler or dust filter at a temperature of 250xc2x0 C. to 550xc2x0 C.
Various policies have been devised to reduce the formation of highly toxic dioxins in an incinerator plant as much as possible. To inhibit dioxin emission into the environment, techniques have been devised to improve incineration conditions and remove dioxin efficiently. However, much time and effort were needed to develop this inhibition technology. Specifically, garbage was incinerated under certain conditions, the concentration of dioxins in flue gas or ash under these conditions was determined, a correlation between combustion conditions and dioxin amount was found, and optimum incinerator conditions or dioxin removal conditions were then found from this correlation.
To make an accurate measurement of dioxin concentration, reference must be made to the regulatory law concerning the assay of dioxins. Generally, quantitative analysis of dioxins is carried out by the technique shown on pp. 441-444 of Pharmacia Vol. 34, No. 5 (1998). This is done by complex pre-processing to separate only desired components from a sample taken from an incinerator under fixed conditions, and performing qualitative and quantitative analysis using a costly, large-scale high resolution mass analyzing device (having a mass resolution of 10000 or more) installed in special equipment which does not release dioxins outside the system.
At the same time, many observation monitors are installed in various parts of an incinerator such as a garbage incinerator to control its operation while it is running. These include monitors for monitoring the temperature of various parts of the incinerator, an oxygen concentration monitor, a carbon monoxide monitor, a nitrogen oxide (NOx) monitor, a sulfur oxide (SOx) monitor, etc. These monitors are used for monitoring and controlling combustion, but they may also be used indirectly as monitors for reducing dioxins as stated on pp. 89-92 of Waste Incineration Technology (Ohm Co., 1995). Specifically, oxygen monitors, carbon monoxide monitors and temperature monitors are observed so that flue gases are completely burnt, and formation of dioxins is inhibited as far as possible.
To monitor the operation of an incinerator, an alternative method has been proposed wherein, instead of attempting to measure the concentration of dioxins directly which may be present in only very low concentrations, another substance present in relatively high concentration is measured, and the concentration of dioxins is estimated from the result. Examples of this technique and devices employing it are given in Yokohama National University Environmental Research Abstracts (Vol. 18, 1992), Japanese Patent Laid-Open No. Hei 4-161849, Japanese Patent Laid-Open No. Hei 5-312796, Japanese Patent Laid-Open No. Hei 7-155731, Japanese Patent Laid-Open No. Hei 9-015229, and Japanese Patent Laid-Open No. Hei 9-243601.
In the technique disclosed by Yokohama National University Research Abstracts (Vol. 18, 1992), Japanese Patent Laid-Open No. Hei 4-161849, and Japanese Patent Laid-Open No. Hei 5-312796, chlorobenzenes are measured by gas chromatography (GC), and are used as indicator for dioxins. The dioxins are estimated from the correlation between the two.
In the technique shown in Japanese Patent Laid-Open No. Hei 7-155731, dioxins in combustion ash are thermally decomposed by heat treatment of the ash, and dioxins are thereby inhibited. Chlorobenzenes or chlorophenols present in the ash before and after heating are analyzed, and a dioxin elimination factor is estimated. In this way, thermal decomposition conditions can be optimized.
In the technique shown in Japanese Patent Laid-Open No. Hei 9-015229, the concentrations of chlorobenzene and chlorophenol in flue gas are measured, and the dioxin concentration is found from this together with a dust concentration and flue gas retention time which are measured separately.
In the technique shown in Japanese Patent Laid-Open No. Hei 9-243601, chlorobenzenes and chlorophenols in flue gas are measured in real time, and the dioxin concentration is measured continuously. The concentrations of chlorobenzenes and chlorophenols are found by leading flue gas into a laser ionization mass spectrometer, ionizing the gas and performing a mass analysis. As a result, the dioxin concentration is found indirectly.
It was hoped that the formation of dioxins and their emission from garbage incineration plants would be reduced by these attempts to improve combustion conditions or use of eliminating techniques. However, it is necessary to measure, in real time, how much dioxin has actually been reduced by adoption of these curtailment policies. From this viewpoint, the following problems are inherent in the conventional methods mentioned above.
Although precise analytical results for dioxins, including types of isomers and their amount, can be obtained from the mandatory methods for its assay, the analysis itself is extremely complex. In addition, special equipment to avoid releasing dioxins outside the system and a costly, bulky, high resolution magnetic mass spectrometer are necessary, and skilled measurement techniques are required. Consequently, the analysis of dioxins cannot be conducted in the incineration plant xe2x80x9con sitexe2x80x9d, which meant that ash samples or gas samples had to be sent to an analysis center, the analysis took almost a week, and the cost involved per sample was of the order of several hundred thousand yen.
There is very little correlation between the numerical values obtained by observation monitors currently employed to control the operation of garbage incinerators, such as oxygen monitors, carbon monoxide monitors, temperature monitors, nitrogen oxide (NOx) monitors and sulfur oxide (SOx) monitors, and the concentration of dioxins. Therefore, it was impossible to know whether or not emission of dioxins was being suppressed, or how much dioxins were being discharged, while an incinerator was operating. Hence, from these indirect monitors, even an estimate of dioxin concentration could not be obtained.
In the techniques indicated by Yokohama National University Environmental Research Abstracts (Vol. 18, 1992), Japanese Patent Laid-Open No. Hei 4-161849 and Japanese Patent Laid-Open No. Hei 5-312796, a minimum of 30 minutes to 1 hour is needed for measurement apart from trapping and concentration time. Moreover, it was difficult to selectively detect chlorobenzenes in the organic compounds which are present in large quantities in flue gas, and there was also a possibility of erroneous measurements due to interfering substances.
In the technique disclosed by Japanese Patent Laid-Open No. Hei 7-155731, specific techniques such as for as on-line sample introduction and automatic measurement are not described, and the measurement itself relied on conventional methods such as GC which required about 20 or 30 minutes per sample apart from the extraction operation.
In the technique disclosed by Japanese Patent Laid-Open No. Hei 9-015229, a clear basis is not given for the relation between dioxins, chlorophenols and chlorobenzenes which is assumed in the invention, and the determination of chlorobenzenes and chlorophenols was performed by the conventional methods which take time such as gas chromatography.
The technique shown in Japanese Patent Laid-Open No. Hei 9-243601 discloses the possibility of real-time concentration measurement of chlorobenzenes, but in this multiphoton ionization, there is said to be a decrease of sensitivity of from 1/7 to 1/10 for each additional chlorine atom substituted in the benzene nucleus. Trichlorobenzene is ionized with an efficiency of only about 1/100 of that of monochlorobenzene, i.e., it can be said that the sensitivity to trichlorobenzene is only 1/100 that of monochlorobenzene. 2,3,7,8tetrachlorodibenzene-p-dioxin (2,3,7,8-TCDD), which is known to be the most toxic dioxin, is a dioxin wherein four hydrogens at positions 2, 3, 7 and 8 are replaced by chlorine. Moreover, all other toxic dioxins are compounds substituted by four or more chlorine atoms. If this highly toxic dioxin is synthesized from chlorobenzenes and chlorophenols, a chlorobenzene or chlorophenol with two, three or more chlorine atoms must be the precursor material. However, in the multiphoton ionization described in this publication, it is difficult to efficiently ionize polysubstituted chlorine compounds. In other words, it was extremely difficult to measure organochlorine compounds such as chlorophenols at a concentration of 1000 ng/Nm3 which are said to be present in incinerator flue gas.
In order to solve the above-mentioned problems, this invention provides a monitor equipped with a sampling system for sampling flue gas and the atmosphere, an ion source for ionizing trace compounds in the sample gas at atmospheric pressure or a pressure close to atmospheric pressure, a mass analyzing part for mass analysis of ions produced by this ion source and measuring its ion current, and a data processor for processing measured signals. As ions can be detected rapidly by mass analysis, monitoring can be performed in real-time.
In this monitor, the tendency of dioxins, chlorobenzenes and chlorophenols to form negative ions is fully exploited. First, sample gas containing hydrocarbon molecules from the incinerator is led to the ion source using a pipe which is heated to prevent adhesion. In this ion source, trace compounds in the sample gas are selectively ionized by a negative corona discharge under a predetermined pressure to form negative ions. These negative ions are analyzed by mass spectrometry, then a qualitative determination of the sample gas is performed from the mass numbers of the observed ions and a quantitative determination of the sample gas is performed from the ion amount. In atmospheric pressure chemical ionization, unlike the conventional laser ion method, the ionizing efficiency does not depend much on the number of chlorine atoms, so dioxins, or dioxin precursors (chlorobenzenes or chlorophenols) with different numbers of chlorine atoms can be detected with high sensitivity. This means that components such as dioxins or dioxin precursors can be monitored. Further, a negative ion is formed by deprotonation from the dioxin precursors in flue gas. This negative ion is introduced to a three-dimensional quadruple mass spectrometer to form a negative ion wherefrom one chlorine atom has been eliminated. The method makes it possible to predict the dioxin generation amount by selectively performing mass analysis on particular decomposition products from dioxin precursors in the negative ions which are produced.
Therefore, dioxins or dioxin precursors present at low concentrations in flue gas can be detected with high sensitivity and rapidity using an ion trap mass spectrometer which traps the ions using a high frequency electric field, and then performs mass analysis.